Colour images are typically printed using three or more colours and black. These colours are known as process colours. Common process colours comprise cyan, magenta and yellow. Black is also used to improve the printing of dark areas. A large range or gamut of colours can be reproduced by printing the process colours in different combinations, together with black. Some printing techniques use other colours either in addition or in substitution for cyan, magenta, yellow and black.
A given colour is reproduced by adjusting the amount of each of the process colours and black. This is achieved by adjusting the ratio of ink to white paper for each process colour and black. Generally the ratio of ink to white paper is controlled by printing each process colour in a series of dots that vary in either size or frequency. The pattern of dots for each process colour is called a halftone pattern, a halftone screen or a screen.
The combination of the individual halftone screens for each process colour and black results in the appearance of the full colour image.
In traditional amplitude modulated screening, the ratio of ink to white paper is controlled by varying the size of individual dots. The dots are arranged on a grid that is defined by the number of lines per inch. This is known as the screen ruling or screen frequency. The grid can be either orthogonal or non-orthogonal.
The grids for each process colour and black will generally be on different angles known as screen angles. In irrational screening, which is a commonly used technique, three orthogonal grids of the same screen ruling are rotated to intervals of 30° relative to each other. The three grids typically represent cyan, magenta and black respectively. A fourth screen for the yellow colour may be of a slightly different screen ruling or frequency which is generally arranged at 15° to two of the other screens. This arrangement results in a pattern that incorporates a series of circular features known as rosettes.
An individual grid comprises an array of cells known as dot cells. Each dot cell generally contains a maximum of one dot. In the case of an orthogonal grid, the dot cells are square.
There are a variety of dot shapes that are used to create halftone screens. Each dot shape is made up of n individual shapes that begin with a white dot cell representing 0% tone. The dot shape is formed and increases in size as the tone darkens (the level of density increases). Eventually a dot cell is formed that is completely filled with ink representing the 100% tone, i.e., the dot shape covers 100% of the area of the dot cell. As the area covered by the dot increases, the density of ink is said to increase or, alternatively, the tone is said to become darker. Often n will be 255 since this number of individual dot shapes, also representing the number of density levels, is an industry norm.
Three traditionally used dot shapes are round dots, elliptical dots and square dots (usually found on a 45 degree orientation where the sides of the dot are orientated at 45 degrees to the edge of the dot cell. Another traditionally used dot shape is the inverse round dot, see for example U.S. Pat. No. 5,680,222. All of these dot shapes have a tendency, when used in a number of screens to make colour images, to result in rosettes that are apparent, particularly at lower screen rulings. The apparentness of rosettes makes the image appear less smooth than an image with less apparent rosettes. The apparentness of the rosettes can vary according to the precise registration of the screens in the image. Precise registration can be difficult to control in practice. The large open white spaces between dots of the traditionally used shapes contributes to the apparentness of the rosettes.
It is an object of preferred embodiments of the present invention to address some of the aforementioned disadvantages of the traditionally used dot shapes. An additional and/or alternative object is to at least provide the public with a useful choice.